1. Field of the Invention
The present invention relates to a wireless personal area network (WPAN). More particularly, the present invention relates to a method of beacon exchange between devices with asymmetric links and a system using the method in the WPAN which may efficiently reduce beacon slot collisions and distributed reservation protocol (DRP) collisions.
2. Description of the Related Art
As it is known to a person working in this field, wireless personal area networks are defined to operate in the personal operating space, i.e. in a range of approximately 10 meters. Some of the protocols defining the physical and data link layer of wireless personal area devices are governed by Institute of Electrical and Electronics Engineering (IEEE) Standards Board. Ultra Wide Band (UWB) technology can provide data rates exceeding several hundreds of Mbps in this personal operating space.
In wireless personal area networks, the medium is shared between all the devices for communication with each other. It necessitates that a medium access control (MAC) mechanism for the devices should be available to manage medium access, broadly including how it may join the network, how each device provides its information to the network, how it can transfer data at the required rate to another device, how the medium is best used and so on. Medium access control for wireless personal area networks can be designed in two approaches—centralized and distributed. In the centralized approach, one of the device acts on behalf of the whole network to coordinate in managing the media access operations for all the devices, and all other devices seek help of the centralized coordinator for media access operations like joining the network, reserving channel time and so on. In the distributed approach, the media access operations are distributed evenly across all the devices in the network and they share the load of managing media access operations for each other.
A wireless personal area network does not have any centralized coordinator, and the devices use the distributed approach for medium access control. All devices cooperate and share information with each other to perform media access control tasks such as allowing a new device to join, allocation of channel time to a device to transmit data to another device, synchronization mechanisms and so on. A distributed WPAN system is formed in an ad-hoc fashion, where devices become active and start broadcasting their identification and capabilities by means of a control frame known a the beacon. FIG. 1 gives a simplistic representation, where all devices are assumed to have the same transmit capabilities. Hence their range is shown to be the same. However, this assumption is not always valid, since devices can have varying capabilities and varying ranges for their communications.
The distributed media access control approach relies on a timing concept called the superframe. A superframe has a fixed length in time and is divided into a number of time windows, which are called time slots. Some of the time slots are used by the devices to send their beacons and the others are used by the devices to send the data. The slots in which beacon is sent are called beacon slots and the slots in which data is sent are called data slots. The length of a beacon slot can be less than the length of a data slot. The beacon slots typically appear together at the start of the superframe. In addition, the number of beacon slots may be fixed or variable, leading to different configurations of distributed Medium Access Control mechanisms.
A superframe consists of several Medium Access Slots, e.g. 256 Medium Access Slots (MAS). Some Medium Access Slots constitute a beacon period which comprises beacon slots used by multiple devices to send the beacon frame, and the remaining MASs constitute a data period, which may be used by different devices in the network to transmit data to other devices in the network. Typically, the superframe duration is 64 milliseconds and each MAS is of 256 microseconds duration. It is important to note here that the channel time allocation schemes and beaconing schemes should be independent of the actual values of these parameters. Information about the device's characteristics and its usage of the superframe is being broadcasted by each device in its beacon frame sent during the beacon period, so that the neighbors of the device can use that information for further processing. The start time of the superframe is determined by the beginning of the beacon period and defined as the beacon period start time (BPST). It is important to note here that the channel time allocation schemes should be independent of the actual values of these parameters. The MAC layer in each device maintains information about the device's individual neighborhood. The neighborhood of each device is defined by its transmission and reception range. All MAC protocol mechanisms are expressed with respect to this individual neighborhood.
MAC protocol algorithms attempt to ensure that a device's identification and the beacon slot in which it transmits its beacon are kept unique. All devices that beacon in one beacon period are defined to belong to a beacon group. A device may not hear the beacon of all the devices in its beacon group. However, the individual neighborhood information of each device is included in its beacon, and hence devices can still get information about other devices which are two hops away.
In the present Multiband OFDM Alliance (MBOA) MAC version 0.98 protocol, as well as in other wireless MAC protocols, the devices in the network are assumed to have identical transmit and receive capabilities. Hence the range of each device in the network is assumed to be the same. Every link in the wireless network is considered to be bi-directional. However, in a typical WPAN or WLAN, heterogeneous devices make up the network. The beacon frame has to be transmitted by all the devices using a specified maximum power so that all devices within the range of 10 meters will be able to receive the beacon and update their neighbor table. However, some devices like headphones, microphones and the like may have limited power capability and may not be able to transmit in the specified maximum power, hence the range of their transmissions is reduced. The beacon sent by such low power devices is not heard by some normal power devices; however, the beacon sent by normal power devices is heard by low power devices.
The qualification “low power device” in this context is used to define devices that are capable of only using low transmit power to transmit its beacon frame and other frames. It does not refer to devices that intentionally reduce the transmit power, nor does it indicate devices that are low in their battery power. Device N is a device with normal transmit power capabilities but device L can only transmit at a lower power. Hence, though the beacon of N reaches L, the beacon of L does not reach N. Then the link between N and L is said to be asymmetric.
Single hop bidirectional communication is not possible between two devices that have an asymmetric link between them. Since the beacon of L does not reach device N, some of the neighbors of N could reuse the beacon slot or data slots used by device L. This could lead to beacon slot collision and DRP collision problems at L. There is no mechanism by which L can communicate the reservation done for its applications to the devices with which it has an asymmetric link. This can cause starvation for resources and unfairness in the use of the wireless medium at the low power devices. In the current MBOA MAC protocol version 0.98, no mechanism is provided to solve the problems that arise due to asymmetric links.
Multi-hop hybrid networks may help by providing both high bandwidth and broad coverage for wireless data networks. The multi-hop hybrid networks focus on Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)-based networks and take IEEE 802.11 as a concrete example and show that the three fundamental operations of synchronization, routing and energy saving can be implemented in an integrated way. The integrated solution is based on the periodic computation of a connectivity tree among the nodes reporting to the same Access Point, starting from the Access Point itself. The nodes that are tree vertices as relays for both data and control packets (also referred to as beacons) are employed in the system, and it establishes a distributed neighbor discovery protocol and a simple centralized algorithm for computing the connectivity tree. The analysis and simulation results show that the proposed solution has low protocol overhead in terms of message passing and execution time, and it performs well even if nodes are mobile.
Thus, the above-described method is employed for periodic computation of a connectivity tree among the nodes reporting to the same Access Point, starting from the Access Point itself. It describes the use of the nodes in tree vertices as relays for both data and control packets (beacon). This multi-hop communication helps reduce energy consumption of the mobile nodes, and allows a lower interference and an increased coverage. However, the method does not describe the use of nodes as relays in cases of asymmetric links.
Accordingly, there is a need for an improved method of beacon exchange between devices with asymmetric links and a need for an improved system for using the method of beacon exchange between devices with asymmetric links.